Conventional telephone networks, such as the Public Switched Telephone Network (PSTN), were initially designed to carry low bandwidth, analog voice communication signals. While the human ear can distinguish audio signals having a frequency as high as 20 kHz, the PSTN was designed to carry audio signals between 300 and 3.4 kHz, which is considered to be sufficient to permit a listener to both understand a voice and to recognize the speaker. At the central office of a telephone network, the analog voice signal is generally digitized into an 8-bit signal at a sampling rate of 8 kHz to provide a 64 kbit/s data stream. Thus, according to the Nyquist theorem, any components of the voice signal above 4 kHz may not be faithfully transmitted by the phone network. In fact, signal components above 4 KHz are filtered out of the voice signal in order to reduce problems such as aliasing.
The local loop between a PSTN central office and a subscriber terminal typically uses twisted-pair wiring, which is capable of carrying voice signals over reasonable distances. In fact, twisted-pair wiring is capable of carrying frequencies well above those required for analog voice communications. Depending on the length and/or quality of the loop wiring, signals having a bandwidth in the tens of megahertz may be carried for short distances over the local loop wiring.
Recently, there has been an expansion of demand for high-speed data communications services, such as broadband internet access, for residential and business subscribers. Digital Subscriber Line (DSL) technology has been developed to provide such data communications services. DSL refers to a family of technologies that provide high speed digital data transmission over conventional telephone wires, such as the twisted-pair wiring of a local telephone network. DSL takes advantage of the unused bandwidth of the local loop by defining a plurality of 4312.5 Hz wide channels starting between 10 and 100 kHz, depending on how the system is configured. The number of channels used depends on the particular implementation of DSL. Each channel may be evaluated for usability. Thus, more usable channels results in more available bandwidth. The pool of usable channels is allocated between upstream and downstream traffic based on system design requirements.
At the central office or at a neighborhood Serving Area Interface (SAI), a Digital Subscriber Line Access Multiplexer (DSLAM) receives DSL signals from multiple subscriber DSL connections. The DSLAM multiplexes the signals onto a high-speed backbone line.
Customers connect to the DSLAM through DSL modems or DSL routers, which are connected to the PSTN network via unshielded twisted-pair telephone lines. A DSLAM may have multiple aggregation cards, each of which may have multiple ports to which subscriber lines are connected. A single DSLAM aggregation card typically has 24 ports. A typical DSLAM may have, in total, up to 192 or more DSL ports.
The quality of the DSL link to the DSLAM may have a direct impact on the number of usable DSL channels available to a subscriber; and thus may directly affect the bandwidth available to a particular subscriber. Thus, it is important for service providers to be able to accurately and reliably identify, report, and respond to errors on a DSL line. Line errors may result in signal corruption that is sometimes referred to as a “code violation.” A code violation may result in an unrecoverable error in a frame of data sent over a DSL line.